Safety interlock



April 1961 H. L. VILKAITIS 2,978,034

SAFETY INTERLOCK Filed Oct. 21, 1958 FIG. I

" INVENTOR.

3 HUGO L. V/L m4 lT/S I /6 a7 33 ATTORNEYS United States Patent SAFETY INTERLOCK Hugo L. Vilkaitis, Thomaston, Conn., assignor to Safeguard Manufacturing Company, Woodbury, Conn., a corporation of Connecticut Filed Oct. 21, 1958, Ser. No. 768,594

Claims. (Cl. 192-131) This invention relates to safety devices which control the operation of a machine and more particularly to a safety interlock requiring the presence of the operator or a portion of the operators body in a specific place before the machine can be operated.

It is an object of this invention to provide a safety interlock which is responsive to direct contact with the human skin to control the operation of a machine.

It is a further object of this invention to provide a reliable safety interlock that utilizes the operators body as part of an electrical circuit that controls the operation ofa machine and to do so in such a way as to avoid discomfort and possible injury to the operator by its use.

It is a still further object to provide a safety device which is inexpensive to manufacture, extremely reliable and foolproof in operation.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly consists in the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application of which will be indicated in the appended claims.

In the drawings:

Fig. l is a schematic side view of a press utilizing a preferred embodiment of this invention;

Fig. 2 is a perspective view of a pair of wristlets usable with the preferred embodiment;

Fig. 3 is a perspective view showing an alternate embodiment of a wristlet usable with this invention; and

Fig. 4 is a block diagram of an' electrical circuit usable with this invention.

Referring now to Fig. 1, it is seen that a punch press generally designated as 1 is provided with a poweroperated reciprocating slide 2 which engages a workpiece (not shown) on table 3 when the operator depresses treadle 4 to actuate the drive mechanism. Support pedestal 5 is mounted in front of press 1 and is provided with inwardly extending arm 6 which pivotally supports lever 7 at point 8. Lever 7 is pivotally connected to slide 2 by link 9 at one end and is secured to steel cables 10 and 11 at its other end so that depression of slide 2 pivots member 7 upwardly to draw cables 10 and 11 away from the punch press. Under normal conditions, wristlets 12 and 13 are secured to cables 10 and 11 and are firmly strapped on the wrist of the operator so that depression of treadle 4' operates slide 2 to forcibly withdraw the hands of the operator from the work area in the event that the operator himself fails to do so.

This safety system suffers from the difiiculty that the operator can merely fail" to wear the wristlets, thereby defeating the safety features of the machine while subjecting himself to possible serious injury. To prevent such unsafe operation of the machine and in accordance with the teachings of this invention, applicant provides a safety interlock which prevents operation of punch press ICC 1 unless wristlets 12 and 13 are properly worn by the operator.

Referring now to Figs. 1 and 2, it is seen that wristlet 12 is provided with an electrode 15 on its inner face so that when the wristlet is strapped in place electrode 15 engages the normally moist skin of the operator. Cable 10 is attached to wristlet 12 and carries therewith conductor 16 which leads to an external circuit subsequently described. Wristlet 13 is similar to wristlet 12 and is provided with an electrode 17 which is connected to an external circuit by conductor 18 which can be carried by cable 11. As most clearly seen in Fig. 2, wristlets 12 and 13 are bands of leather which adjustably engage the operators wrist through the action of buckle fasteners 19. As hereafter described more fully, electrodes 15 and 17 are made from dissimilar metals.

Referring now to Fig. 3, it is seen that an alternative embodiment of applicants invention utilizes a single wristlet in which two electrodes 20 and 21 are carried by the adjustable leather band or wristlet 22. Connection to the external circuit is established by conductors 23 and 24 and attachment to a safety cable 25 can be established by eyelet 26 fastened to the wristlet.

Referring now to the drawings in general and particularly Figs. 1 and 4, it is seen that electrodes 15 and 17 are connected to amplifier 30 through wires 16 and 18. It has been found that it is not necessary, however, to utilize separate conductors since the steel pull on cables can be used to transmit the signal. Amplifier 30 is of any common type, the only requirement being that it be capable of sensing the potential between elec trodes 15 and 17 and producing an output responsive to this potential which is of a sufiicient amplitude to operate a safety device. Amplifier 30 is energized from a suitable power source 31 through amplifier power sup ply connections 32 and 33. The output of amplifier 30 is connected to safety device 35 through connections'36 and 37. As most clearly seen in Fig. 1, the safety device generally designated as 35 in Fig. 4 is a solenoid 38. Solenoid 38, when energized, withdraws locking pin 40 from notch 41 in operating bar 42 thereby releasing treadle 4 for movement to operate punch press 1.

In operation, electrodes 15 and 17, when strapped on the wrists of the operator, establish a low potential between them because of the electrolytic action experienced when dissimilar metal electrodes are placed in contact with an electrolyte. The saline nature of moisture normally present on the surface of human skin acts as the necessary electrolyte in this invention. This potential difference will produce a minute current in the input of amplifier 30 which is amplified to actuate the safety device 35 (relay 38 in Fig. 1). The selection of metal for electrodes 15 and 17 can be made from the Table of Electrode Potential Series as set forth in International Critical Tables, volume 6, McGraw Hill Book Company, 1929. Any two metals from this series will exhibit a potential difference between them when in contact with an electrolyte. It has been found, however, that zinc and silver provide the best operating electrodes since they are reasonably inexpensive, not subject to excessive body action corrosion and produce a substantial potential difference by comparison between them. It has also been found that additional metals such as aluminum, gold and rhodium can be utilized with reasonable success.

Fig. 3 illustrates a wristlet containing two electrodes thereby permitting one-hand closure of the interlock circuit to control the operation of a machine. Fig. 3 also most clearly illustrates the use of mesh as the electrode. It has been found that the use of a mesh electrode not only permits intimate contact with the skin, but also can be used to greatly improve the mechanical strength of the 3 wristlet as, for example, by fastening the pull out cable 25 to the mesh.

It is therefore seen that this invention provides a reliable safety interlock that is sensitive to contact with human skin and that is ideally suited for use in a great many safety applications where reliability, comfort to the user, and size are of great importance.

As will be apparent to persons skilled in the art, various modifications and adaptations of the structure above describedwill become readily apparent without departure from the spirit and scope of the invention, the scope of which is defined in the appended claims.

l lai 1. A safety device for power operated machines comprising a plurality of dissimilar metal electrodes, means for positioning said electrodes in contact with the normally moist skin of the machine operator thereby to establish an electrolytic potential between the electrodes, means electrically connected to said electrodes for amplifying the electrolytic potential established therebetween,

and means for controlling the operation of the machine, said last named means being operable by the output of said amplifier thereby to permit machine operation only when said dissimilar metal electrodes are in engagement with the skin of the operator.

2. A safety device for power operated machines comprising a plurality of dissimilar metal electrodes, a wristlet for positioning said electrodes in contact with the normally moist skin of the machine operator thereby to establish an electrolytic potential between the electrodes, means electrically connected to said electrodes for amplifying the electrolytic potential established therebetween,

and means for controlling the operation of the machine, said last named means being operable by the output of said amplifier thereby to permit machine operation only when said dissimilar metal electrodes are in engagement with the skin of the operator.

4 3. A safety interlock for presses and the like having a pair of wristlets that are, cable actuated by the press to withdraw the hands of the operator from the press operating area, said interlock comprising a first electrode positioned in one of the wristlets and engageable with the skin of the operatona second electrode positioned in the other of the wristlets and engageable with the skin of the operator, said first and second electrodes being made of dissimilar metals, an amplifier having input terminals connected to saidelectrodes thereby to amplify the electrolytic potential produced by contact of the electrodes with the normally moist skin of the operator, :1 safety device operable to permit press operation and means connecting the output of said amplifier to said safety device thereby to prevent operation of the press unless the electrodes carried by the wristlets are in engagement with the skin of the operator.

4. A safety device as set forth in claim 1 wherein each of said electrodes is made from a flexible metal mesh.

5. The safety device as set forth in claim 1 wherein said plurality of dissimilar metal electrodes includes at least one electrode made of zinc and a second electrode made of silver.

References Cited in the file of this patent UNITED STATES PATENTS 371,553 France Jan. 26, 

